Phase Transitions and Coexistence of Magnetic and Electric Orders in the Methylhydrazinium Metal Formate Frameworks

Abstract

We report the synthesis of four perovskite-type metal formate frameworks, [CH3NH2NH2][M(HCOO)3] (MHyM) with M = Mn, Mg, Fe, and Zn. These compounds exhibit two structural phase transitions. The first transition temperature depends weakly on a type of divalent metal and is observed at 310–327 K on heating. X-ray diffraction, DSC, and vibrational studies revealed that it has a second-order character. It is associated with partial ordering of the methylhydrazinium (MHy+) cations and change of symmetry from nonpolar R3̅c to polar R3c. Pyroelectric measurements suggest the ferroelectric nature of the room-temperature phase. The second, low-temperature phase transition has a first-order character and is associated with further ordering of the MHy+ cations and distortion of the metal formate framework. Magnetic susceptibility data show that MHyMn and MHyFe exhibit ferromagnetic-like phase transitions at 9 and 21 K, respectively. Since the low-temperature phase is polar, these compounds are possible multiferroic materials. MHyFe shows additional magnetic anomaly in the magnetically ordered state, which most likely manifests some blocking of magnetic moments. We also report high-pressure Raman scattering studies of MHyMn that revealed a pressure-induced reversible phase transition between 4.8 and 5.5 GPa. Analysis of the data indicates that the transition leads to significant changes in both the manganese formate framework and the MHy+ structure.